This invention relates to imaging systems, and more particularly, to improved xerographic developing materials, their manufacture, and use.
Electrostatography, that branch of the imaging art which relates to the formation and utilization of latent electrostatic charge patterns to record and reproduce patterns in visible form is well known in the art. When a photoconductor is employed to form these electrostatic latent images by first charging and then selectively exposing the photoconductive layer, this imaging method is referred to as electrophotography and more commonly known as xerography, the basic techniques of which are disclosed in U.S. Pat. No. 2,297,691. The latent electrostatic images thus formed may be developed or rendered visible by deposition of a finely divided electroscopic material referred to in the art as toner. The image thus obtained may be utilized in a number of ways, for example, the image may be fused or fixed in place or transferred and then fixed to a second surface.
Electrography, the other broad general branch of electrostatography, generally divided into two broad sectors which are referred to as xeroprinting and electrographic or TESI recording, does not employ a photoresponsive medium, the charging and selective discharging thereof to form its latent electrostatic image. Xeroprinting, the electrostatic analog of ordinary printing is more fully described in U.S. Pat. No. 2,576,047 to Schaffert. TESI imaging or transfer of electrostatic images, more fully described in U.S. Pat. No. 2,285,814, involves the formation of an electrostatic charge pattern conforming to a desired reproduction on a uniform insulating layer by means of an electrical discharge between two or more electrodes on opposite sides of the insulating medium. The lines of force generated by the latent electrostatic image are employed to control the deposition of the toner material to form an image. Various developers both powder and liquid and developing systems are well known to those skilled in the art including cascade development as disclosed in U.S. Pat. No. 2,618,552 to E. N. Wise; magnetic brush development as generally described in U.S. Pat. No. 2,874,063; powder cloud development as generally described in U.S. Pat. No. 2,784,109; touchdown development described in U.S. Pat. No. 3,166,432; and liquid development as described in U.S. Pat. No. 2,877,133 among others. These development systems, though they enjoy widespread use for black and white reproductions may also be employed in other colors and combinations of colors for example, a trichromatic color system of either the additive or subtractive color formation types. In full color systems at least three different colors must be employed to synthesize any other desired color which involves generally the formation of at least three color separation images and their combination in registration with each other to form a color reproduction of the original. Thus, in any of the electrostatographic recording systems at least three different latent electrostatic images must be formed, developed with different color toners and combined to form the final image. For example, in color xerography an electrostatic latent image resulting from exposure to a first primary color may be formed on the photoconductive layer and developed with a toner complementary to the primary color. In a similar fashion, succeeding developments of electrostatic latent images corresponding to primary colors are accomplished with complementary toners. When exposing through color separation negatives, the toner is the complement of the radiation of exposure.
In a three color electrophotographic system which employs superimposed color images it is necessary that the toners be quite transparent except for the underlying one so as not to obscure the different colored toner images below it and that each toner have sufficient color saturation at the same time and brightness to satisfy the colorimetric requirements for three color synthesis of natural color images. As can be appreciated, these requirements are virtually diametrically opposed and are further complicated by the additional requirement that when all the toners are combined, they must produce a deep black. It has been found that in order to produce deep blacks in a color system it is required to superpose four different colored images including a black registered image. Additional problems generally arise when inorganic pigments are used as the coloring material either in printing inks or electrophotographic toners since it is difficult to achieve proper color balance and saturation while at the same time keeping the colors transparent. When employing inorganic pigments, the range of colors available is relatively narrow and these pigments are found to impart opacity to the materials to which they are added even in relatively small amounts.
Bartoszewicz et al in U.S. Pat. No. 3,345,293 teaches colored electrophotographic toners comprising substantially transparent resin particles containing organic dye pigments. These materials are stated to be advantageous in their use over prior art materials in that they are more resistant to bleeding of color upon toner fusing and they are specifically adaptable for use in three color electrophotographic processes since their colors are yellow, cyan, magenta and their mixtures in pairs produce blue, red and green while the three toners together produce a black. Notwithstanding the apparent advantages of the Bartoszewicz et al toners, there are nevertheless disadvantages connected with these specific toners, specifically in the case of the yellow toner when employed in an automatic electrophotographic machine. The yellow colorant as advanced by Bartoszewicz et al consists essentially of from about 0.92 to about 1.08 parts by weight of 3,3'-dichloro, 4'-bis (2"-acetyl-2"-azo-o-acetotoluidide)biphenyl per 10 parts by weight of a substantially transparent resin. The problem in employing this colorant resides in its inability to disperse substantially uniformly in transparent resin materials and more significantly the undersirable triboelectric properties which result from its use causing poor images of low contrast and low machine life. It is found that the triboelectric properties of the resulting toner material are not maintained under conditions where the toner is exposed to mechanical abrasion, high temperatures, and high ambient humidity conditions, all of which are common in electrophotographic machines. This results in a number of problems including poor transfers from the drum surface to the copy sheet as well as maintaining cleanliness of the drum. More specifically, it is found that in electrophotographic machine use this toner impacts on its carrier further degrading the already existing undesirable triboelectric relationship and thereby adversely effecting machine performance.